As you can see, everything depends on the flow of blood to and from the heart.
So how does the heart do this?
First, the heart, which is a muscle, has 4 chambers.
From one chamber, blood is pumped to the lungs.
This is the blood that carries carbon dioxide.
When the oxygenated blood from the lungs comes back to the heart, it enters another chamber.
This chamber is called the left atrium.
From the left atrium, the oxygenated blood is pumped into another chamber, the left ventricle.
The left ventricle then pumps the blood into the aorta, which is the largest artery in the body.
From the aorta, the oxygenated blood passes through a network of smaller arteries throughout the entire body, including the brain.
Blood returns to the heart through a system of veins and enters the upper right chamber, the right atrium.
Now that we understand a little about the circulatory system we can ask what it needs to operate.
Of course it can't operate without energy.
This energy is supplied by another system, the digestive system, which gets energy from food.
And of course these systems need to be supported and controlled.
Our skeletal system provides support, and our muscular system provides control and movement.
Without support and movement, we couldn't get the food that we need for energy.
Only plants can do this because they have a completely different system, which gets energy directly from the sun.
To summarize, the human body is like a huge, complex factory.
The language we use to describe how it functions is the same language we use to describe almost any industrial process.
The vocabulary is different, but the processes, flows and logical relationships are almost identical.
Think of some familiar process in your life or job and see if you can describe and explain it in English.
（The digestive system supplies energy to the circulatory system.）
（Plants get energy directly from the sun by using photosynthesis.）
